In electrophotographic machines, copies of documents or other subjects are produced by creating an image of the subject on a photoreceptive surface, developing the image, and then fusing the image to copy material. In machines utilizing plain bond copy paper or other ordinary image receiving material not specially coated, the electrophotographic process is of the transfer type where a photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In a typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an imaging station where it receives light rays which may be reflected from a document to be copied or generated by some type of light producing printhead. In the case of reflected light from a document, the white areas reflect large amounts of light thus discharging photoreceptive material to relatively low levels while black areas reflect little light causing the corresponding areas on the photoreceptive material to continue to carry high voltage levels even after the exposure. In that manner, the photoreceptive material is caused to bear a charged pattern which corresponds to the printing, shading, etc. present on the original document.
After receiving the image, the photoreceptor is moved to a developing station where a toning material is placed on the image. This material may be in the form of a black powder which carries a charge opposite in polarity to the charge pattern on the photoreceptor. Because of the attraction of the oppositely charged toner, particles of the toner adhere to the surface of the photoreceptor in proportions related to the shading of the original. Thus, black character printing should receive heavy toner deposits, white background areas should receive none, and gray or otherwise shaded half-tone character portions of the original should receive intermediate amounts.
The developed image is moved from the developer to a transfer station where copy receiving material, usually paper, is juxtaposed to the developed image on the photoreceptor. By placing a charge on the backside of the copy paper and stripping the paper from the photoreceptor the toner material is held on the paper and removed from the photoreceptor. After transfer, the paper is moved into a fuser where the toning material is permanently joined to the paper.
In the developing step outlined above, it has become common in contemporaneous machines for magnetic brush developing components to be used. In the typical magnetic brush developer, a rotating cylinder surrounds stationary magnetic rolls which attract magnetic material to the surface of the cylinder. That material is then carried to the development zone, where an electrical field is present due to the charge on the photoreceptive material. That charge attracts the oppositely charged toner to the surface of the photoreceptor as mentioned above. Since the toner is carried out of the machine on the surface of the paper, it is apparent that toner is a supply item which must be periodically replenished.
Additionally, in developer mixes where the toner is nonmagnetic, it must be mixed with carrier particles which are magnetic and oppositely charged to the toner. In such case, it is necessary to maintain an appropriate concentration of toner particles to carrier particles so that good development of the latent image is obtained.
While many methods for controlling the toner concentration in a developer mix have been invented, a particularly useful toner concentration control scheme is outlined in U.S. Pat. No. 4,183,657. In this scheme, a special test cycle is run wherein the photoconductor is charged by the charge corona to the customary dark voltage level but an exposure is not made at the exposure station. Instead, the interimage and edge erase lamps are utilized to erase all of the charge with the exception of a small stripe or patch that is located in the area of the photoconductor ordinarily used for the production of the latent image. That small patch is then developed at the developer and passed under a toner concentration control station where light rays are directed onto the developed patch and a photosensor senses the degree of reflectivity which results. That degree of reflectivity is then compared to the reflectivity of the bare photoconductor in the undeveloped areas of the latent image area with the result that a measure of the toner concentration on that particular photoconductor is obtained. In that manner, the quantity of toner in the developer mix can be adjusted to keep the toner concentration at a desired level.
Recently, the copier industry has begun to utilize higher optical densities in order to obtain copies with an improved quality appearance. Optical density is a measure of how black the development is and is a logarithmic scale produced from measurements taken with a reflectometer. In order to obtain higher optical densities, the amount of toner in the toner carrier ratio must be increased or different developer mix chemicals used, or multiple pass developer stations used. Whatever the technique of increasing optical density, problems are created in the toner concentration control scheme outlined above since high density development of the control patch produce burdens on the cleaning station and may create a situation where the optical sensor becomes insensitive to changes in toner density. This results if the optical sensor reaches the limit of its ability to sense changes in the optical density. For example, suppose that a control point is established that calls for the developed patch to produce thirty times less reflected light than the bare photoconductor. Suppose further that this ratio produces as black a patch as can be sensed. If during the course of machine use the control is readjusted to a level of, for example, 34 to 1 the sensing mechanism would call for increases in the concentration of toner. However, if the optical sensor is incapable of sensing changes above a ratio of 30 to 1, the desired toner concentration level of 34 to 1 will not be sensed. Consequently, toner will continue to be added into the developer mix and ultimately a level of 40 or 50 to 1 might be reached. As a consequence of too much toner in the developer mix, poor copy quality will result (high background), the cleaning station will be overloaded resulting in poor cleaning, and contamination of the machine will probably result. In order to remedy these problems, the inventors herein have adjusted the magnetic brush voltage level during the development of the toner concentration control patch so that development occurs at a gray level rather than at a black level. In one machine, for example, a gray patch is produced if the developer bias is at 450 volts, whereas black development occurs at 300 volts. In that machine, the required adjustment of developer bias voltage to 450 volts is accomplished during the test cycle.
Unfortunately, while sensing toner concentration levels in the gray area provides for decreased cleaning station burdens and provides for the required optical sensitivity, it produces another problem in that changes in dark voltage produce, proportionately, greater changes in toner concentration for a gray patch than for a black patch. To illustrate, suppose that a particular machine carries a nominal dark voltage of 860 volts, a nominal magnetic brush bias voltage of 300 volts, and a gray brush bias level of 450 volts. These parameters result in a black vector of 560 volts and a gray vector of 410 volts. In this situation, if changes in the charge corona power supply, corona contamination, or changes in the electrostatic sensitivity of the photoconductor result in a change in dark voltage, the sensitivity of the gray vector to that change substantially exceeds that of the black vector. As a consequence, the toner concentration level may be altered more quickly outside of a range of toner concentrations suitable for good development. For example, a particular concentration of 1% toner by weight might produce nominal development and a range of 0.9 to 1.1% might produce similar good development. However, if the patch test results in concentration outside of the appropriate range, development quality may decline or some other deleterious effect may take place, such as machine dusting with toner particles.